1. Field of the Invention
The present invention relates to a circuit, especially to a circuit capable of enhancing the Q factor of an inductor.
2. Description of the Prior Art
A typical inductor in an analog circuit is usually a planar inductor having a Q factor of approximately 3˜8. This low Q factor often limits the application of an inductor. For example, when an inductor is utilized as a load of an amplifier, the gain of the amplifier is proportional to the square of the inductor's Q factor. Therefore, a low Q factor implies a low gain. Moreover, in some narrow-band communication systems, an inductor with high Q factor is utilized for making a narrow-band resonant chamber capable of filtering out interfering signals. The 3 dB bandwidth of the resonant chamber is inversely proportional to the Q factor of the inductor. Therefore, an inductor with a higher Q factor leads to a narrower-band resonant chamber.
Some methods for enhancing the Q factor of an inductor are proposed, such as removing the substrate underneath the inductor to reduce power consumption of the substrate, inserting an isolation metal layer between the inductor and the substrate to reduce power consumption due to a vortex current, and substituting aluminum with high conductance metal (e.g., silver). However, these methods have a significant common drawback of being incompatible with the standard CMOS manufacturing methods. In addition to the methods mentioned above, an inductor can also be implemented by active circuit. However, the noise and linearity of an active circuit limits its application while being utilized in high-frequency and low-noise circuits.